Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. As used herein, a “radio frequency” signal means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. One common type of RF receiver is the so-called superheterodyne receiver. A superheterodyne receiver mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted back down to baseband for further processing. Thus a superheterodyne receiver requires two mixing steps.
One well-known problem with the mixing process is that it creates image signals. For some RF systems the level of the image signal is small enough so that designers can rely on the attenuation characteristics of an RF bandpass filter alone to reject image signals. However for other systems the attenuation of the RF bandpass filter is not sufficient. For example satellite radio uses a 2.3 GHz carrier frequency. Each channel has a baseband spectrum from 1 megahertz (MHz) to about 13 MHz, and the adjacent signal spectra can create large image signals. In these systems additional image rejection filtering is required to maintain a sufficient signal-to-noise ratio (SNR) in the desired signal.
One known image-rejecting mixer uses a tracking bandpass filter to reject the image frequency before the RF signal is input to the mixer. The center frequency of the filter is made to track the local oscillator frequency so that all frequencies outside the desired sideband frequency are attenuated. Then the output of the mixer is subjected to further filtering to reject the image frequency.
Another known architecture uses two mixers in quadrature to produce in-phase and quadrature components of the RF signal. The outputs of the mixers are then input to a polyphase filter. The polyphase filter has the desirable quality that it is asymmetric so that it can pass the desired sideband and attenuate the image frequency. However known mixer architectures using polyphase filters add noise due to the resistors used in the filter. Furthermore to increase the amount of image rejection, higher order polyphase filters may be used. However these higher order filters require additional resistors for each added filter stage, increasing noise. What is needed then is a new image reject filter architecture for RF receivers with high image rejection but low noise.